Alignment microscopes, in particular so-called BSA (bottom side alignment) microscopes are used if substrates or wafers which have their alignment marks on their bottom sides have to be adjusted with respect to exposure masks. The structure of such an alignment system is schematically shown in FIG. 1. Before loading the wafer 2 into the system, the microscope 1 is first focused to the alignment mark of the exposure mask 3 and the image of this mark is centered. The position of the alignment mark is stored, and subsequently the wafer 2 is inserted between the microscope 1 and the mask 3. The microscope is then focused to the alignment mark located on the wafer 2. Finally, the position of the wafer 2 is changed in the focusing plane until the mark on the wafer 2 is adjusted with respect to the mark on the mask 3; for this purpose, the stored position of the mark on the mask 3 is superimposed on a monitor with the current position of the mark on the wafer 2 so that an alignment is possible.
During this adjustment process, the alignment microscope 1 must be strongly refocused. However, this requires that previously the microscope 1 has been adjusted exactly, i.e. that the optical axis 11 of the microscope 1 is as perpendicular as possible to the plane of the mask 3 or wafer 2.
In known methods for adjusting alignment microscopes so-called double mark masks are used. FIG. 2(a) schematically shows such a construction for performing such a known adjustment method. The (transparent) double mark masks 4 comprise alignment marks both on their top surface 42 and their bottom surface 41, e.g. cross structures 44 and 43, respectively, which were adjusted with respect to each other by the mask manufacturer. For adjusting the alignment microscope, a double mark mask 4, instead of the mask 3, is loaded into the alignment system. The alignment is performed in that the microscope is first focused to the alignment mark 43 on the bottom side 41 of the double mark mask. After the microscope 1 has been adjusted so that the alignment mark 43 is in the focus, its position is stored and then the alignment microscope 1 is refocused to the upper side 42 of the mask. The position of the alignment mark 43 stored previously is then compared with the live image of the alignment mark 44. The alignment microscope 1 is adjusted until the alignment marks are superimposed in both images and overlap one another. For simplifying the alignment, the upper cross structures 44 can also be shaped as double lines.
It is extremely difficult to manufacture double mark masks in which the alignment marks on the top and bottom sides of the mask have been adjusted with the required accuracy with respect to each other. The manufacture of such double mark masks is therefore very expensive.
U.S. Pat. No. 6,340,821 describes a projection eyepiece and a method for aligning patterned areas on a substrate surface having a micro-optical device on an opposite surface side of the substrate. In the described method for aligning the alignment microscope and the optical axis of a micro-mirror device, a real image of the reticle projected by the projection eyepiece is compared with the image of the reticle reflected by the micro-mirror after the mask aligner and the mirror on the substrate have been roughly aligned. Further background information can be found in documents DE-A-100 18 810 and DE-C-42 42 632.
It is the object of the present invention to provide an improved method for adjusting alignment microscopes. Moreover, the present invention provides for an improved device for adjusting alignment microscopes which can be manufactured in a simpler and more cost-saving manner but still guarantees the required accuracy in connection with the adjustment of the microscope.
This object is achieved with the features of the claims.